1. Field of the Invention
The present invention relates to a charge-coupled device (hereinafter referred to as "CCD"), and more particularly to a charge-coupled device having at its cell section a photo-shield structure in which a metal wiring formed from a metal film, a refractory metal film or its silicides serves both as wiring for supplying a predetermined potential to charge transfer electrodes constituting a vertical charge transfer section and as a photo-shield for the vertical charge transfer section.
2. Description of the Related Art
The CCD is superior to conventional and generally used image tubes in terms of the size, weight, durability, image residue and burning and, in the field of compact home video cameras, the image tube has already been taken over by the CCD. This trend is also true in the field of industrial cameras in which image sizes are comparatively large.
In the CCD with a comparatively large image size (for example, 1 inch or 2/3 inches), the typical example being that for a high-definition television (HDTV) system, there is a problem with the method for supplying drive pulses from two sides of an image region to a polysilicon transfer electrode. This problem is that, since the polysilicon charge transfer electrode has high resistance and large capacitance, the amplitude of the drive pulses is markedly reduced and the maximum transfer charge amount is reduced at a center portion of the image region especially when the transfer speed becomes high.
With a view to overcome the above problem, there has been proposed "A Frame Interline Transfer CCD Image Sensor for HDTV Camera System" in ISSCC Digest of Technical Papers, pp 88-89, February 1989, in which the metal wiring for supplying drive pulses to the charge transfer electrode constituting the vertical charge transfer section also functions as light-shields or photo-shields for the vertical charge transfer section.
FIG. 1 shows in plan view a portion of the image region of a four-phase CCD used in the above described conventional technique.
First and second vertical charge transfer electrodes 2 and 3 formed from polysilicon and constituting the vertical charge transfer section are connected at a cycle of four picture elements through contact holes 4 with the metal wirings 5 serving also as photo-shields, whereby the wiring resistance of the first and the second vertical charge transfer electrodes 2 and 3 can be made relatively low and the lowering of the amplitude of the drive pulses can be suppressed.
However, in the conventional CCDs having a structure in which a metal wiring formed from a metal film, a refractory metal film or its silicides serves both as wiring for supplying a predetermined potential to charge transfer electrodes constituting the vertical charge transfer section and as a photo-shield for the vertical charge transfer section, the photo-shielding characteristic at a vertical bordering region of a photoelectric conversion section 1 or at a section neighboring such region is poor. Therefore, the signal electron or charge excited by the light incident to such region at a deeper layer section of the photoelectric conversion section 1 in an inactive region or a region neighboring such region leaks into the vertical charge transfer section so that the false signal caused by "smear phenomena" increases and the image signal deteriorates.
If an attempt is made to overcome the above problem by narrowing the spacing between the wirings/photo-shields for the vertical charge transfer section, the sensitivity of the device deteriorates by a proportion inverse to this spacing. This is a problem to be solved by the invention, in the conventional photo-shield structure of the charge-coupled device.